Spring Data JPA在创建对象时保存多对多关系

@Entity
public class Student {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;

    @ManyToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE})
    @JoinTable(name = "student_course",
               joinColumns = @JoinColumn(name = "student_id"),
               inverseJoinColumns = @JoinColumn(name = "course_id"))
    private Set<Course> courses = new HashSet<>();

    // getters and setters
}

@Entity
public class Course {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String title;

    @ManyToMany(mappedBy = "courses")
    private Set<Student> students = new HashSet<>();

    // getters and setters
}

@Service
public class StudentService {

    @PersistenceContext
    private EntityManager entityManager;

    public void enrollStudentInCourse(Long studentId, Long courseId) {
        Student student = entityManager.find(Student.class, studentId);
        Course course = entityManager.find(Course.class, courseId);

        if (student != null && course != null) {
            student.getCourses().add(course);
            entityManager.persist(student); // 或者使用 merge 如果 student 已经存在于数据库中
        }
    }
}

student.getCourses().add(course);
course.getStudents().add(student); // 双向关系需要双方都更新
entityManager.persist(student);